Gaming machines include games of chance such as slot machines. The traditional mechanical slot machine includes three or four symbol bearing reels, which are rotatably mounted on a common axis. The symbols are located on the peripheries of the reels, and are typically pictures of bells, bars, and fruit. There are also "blank" symbols, which are the portions of the reels' peripheries in between the picture symbols. A line (the "win line") is placed adjacent to the reels, so that when the reels are at rest, at least one symbol from each reel is visually associated with the win line.
To play the slot machine, the player spins the reels by pulling a lever which is mechanically linked to the reels. After a brief period of spinning, the reels come to rest, each reel displaying a symbol or blank space along the win line. The displayed combination of symbols is a random game result, and corresponds to a predetermined payout, which may be zero. The payout for a particular game result usually depends on the probability of that game result occurring.
Each reel's final resting position will be one of a predetermined and discreet "reel stop positions." At each reel stop position, a particular part of the reel's periphery (either a symbol or a blank space) is displayed at the win line. Thus, each reel stop is associated with a particular symbol or blank. In a mechanical slot machine, the probability of a particular symbol being displayed at the win line is N.sub.S divided by N.sub.R, where N.sub.R is the total number of reel stop positions, and N.sub.S is the number of reel stop positions associated with the particular symbol. Where a symbol is associated with only a single reel stop position, its probability of being displayed is one in N.sub.R. Thus, the range or "spectrum" of probabilities which can be developed in a mechanical slot machine is 1:1 through 1:N.sub.R.
In the 1970's, manufacturers developed electronic versions of the traditional mechanical slot machine. In these electronic machines, the reels are computer controlled, and there is no mechanical linkage between the lever and the reels. Instead, when the user pulls the lever, the computer randomly selects reel stop positions for each of the reels, and then sets the reels into motion with a motor. The reels are allowed to spin for a short time, and then are stopped at the selected reel stop positions.
In effect, the game result is determined by the computer, with the spinning reels used only to display that result. Thus, in some machines, the reels are eliminated altogether, and the game result displayed on a video screen. The video display is often a representation of spinning reels, to preserve the charm and excitement of the traditional slot machine.
In determining a game result, the computer simulates the mechanical slot machines by randomly picking reel stop positions for each reel. A table in the computer's memory indicates which symbol (or blank space) is associated with each reel stop position, so the computer can determine the game outcome (that is, the ultimate combination of selected symbols).
In a mechanical reel slot machine, the spinning reel is equally likely to come to rest at one reel stop position as another. Thus, each reel stop position has an equal chance of being "selected." This is referred to as a "uniform probability distribution." For example, in a three-reel uniform probability machine with thirty-two reel stop positions, the lowest possible probability for a particular game outcome is one chance in 32.sup.3 (or 1:32,768). Assuming each play costs one dollar, the payout for this particular game outcome cannot exceed $32,768, without the game losing money over time to the players.
In an electronic slot machine, the computer can also pick reel stop positions in accordance with a uniform probability distribution. Alternatively, the computer can assign different probabilities to different reel stop positions. This is referred to as "nonuniform probability distribution." The advantage to nonuniform distributions is that they allow the spectrum of game result probabilities to be greatly expanded. Thus, in a nonuniform probability system, certain game outcomes can be assigned low probabilities, such as, for example, one in one million. The corresponding payout can be increased without making the machine unprofitable--in this example, the payout could be one million dollars (assuming a one dollar bet). These high payouts, although extremely rare, are attractive to many players, and therefore are a desirable feature to have on a gaming machine.
One way in which expanded probability spectrums have been implemented in slot machines is by using a "virtual" reel. A virtual reel is a model of a physical reel which exists only in the computer's memory. The virtual reel can have a large number of reel stop positions--far more than a physical reel. Each reel stop position in the virtual reel is associated with a particular symbol. Symbols corresponding to higher payouts are associated with only a few (or even one) virtual reel stop position. Thus, the probability of a game outcome including such symbols is greatly reduced. Because the virtual reel has more reel stop positions than a physical reel, its probability spectrum is increased.
Another technique for expanding the probability spectrum in gaming machines is the "time based" method. In the time based method, game outcomes are represented by the contents of a digital counter or other suitable state machine. The counter has a range of zero to seven, for example, and each of its eight possible values corresponds to a game event. The counter rapidly and repetitively cycles through its range. At an arbitrary point in time, a player presses a button and interrupts the counter, leaving it suspended on a particular number. This number is random in the sense that it can not be predicted by the player, and the event corresponding to this number is selected as the game outcome.
It has been recognized that the odds of selecting a particular number (that is, game outcome) can be varied by adjusting the relative amount of time that the counter holds each number. Thus, if the counter holds one number longer than the other, it is more likely to be holding that number when it is interrupted by the player. Likewise, if the counter spends less time holding a particular number, then it is less likely that the counter will be holding that number when it is interrupted by the player.
To vary the time which the counter spends at each number, the counter is driven by a variable frequency astable multivibrator. Each cycle of the multivibrator generates a pulse, which increments the counter. The duration of the period between pulses is controlled by a series of RC networks, each having a different resistance value. The networks are successively electronically coupled to the multivibrator each time the counter is incremented. Thus, the intervals between pulses (and, consequently, the amount of time the counter spends at each number) vary in accordance with the value of the resistor in the particular RC network which is coupled to the multivibrator.